1. Field of the Invention
The present invention relates to methods of cementing casing strings in well bores, and more particularly, to methods of cementing conductor or surface casing strings in deep water offshore wells.
2. Description of the Prior Art
In carrying out completion operations in oil and gas wells, hydraulic cement compositions are commonly utilized. For example, hydraulic cement compositions are used in primary cementing operations whereby casing strings are cemented in well bores. That is, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior of a casing string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened impermeable cement therein. The objective of the cement sheath is to physically support and position the casing string in the well bore and bond the casing string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
Primary cementing operations in deep water offshore wells are particularly difficult in that they are carried out in well bores which penetrate formations between the sea floor or mud line and a depth generally under about 2,000 feet below the mud line. Such formations are often not well consolidated, readily fracture and often have highly pressured water flows therethrough. For example, the fracture gradients in subsea well bores in which conductor and/or surface casing strings are cemented limit the densities of the cement compositions utilized to 11 or 12 pounds per gallon. The high water flows through the well bores often wash away cement compositions which have densities below 10 to 11 pounds per gallon.
Another problem involved in deep water offshore well cementing is the temperature at which the cement composition must set. Deep water offshore wells typically have sea bottom temperatures ranging from about 32.degree. F. to 55.degree. F. depending on the geographical location. The cement compositions utilized for performing cementing operations at such temperatures must set and provide enough compressive strength to proceed with drilling without involving long waiting-on-cement (WOC) times, preferably less than 24 hours. Accordingly, the cement compositions must include set and strength accelerating agents to allow the cement compositions to set at the low temperatures involved and develop early compressive strengths. However, a problem in the use of set and strength accelerating agents is that they often cause the cement compositions to have thickening times which are too short to allow placement of the cement compositions in the formations or zones to be cemented. Thus, the cement compositions used in deep offshore wells must have adequate pumping times to allow placement, but at the same time they must set and develop sufficient compressive strengths to allow further drilling as quickly as possible. The generally accepted requirements for cement compositions to overcome the above described problems in the Gulf Coast region of the United States include cement composition densities in the range of from 10 to 12 pounds per gallon, thickening times of from 3 to 5 hours and compressive strengths of from 400 to 600 psi at temperatures of from about 45.degree. F. to about 55.degree. F.
Foamed cement compositions have heretofore been utilized in deep water offshore wells to obtain the low densities required. Set and strength accelerating agents such as calcium chloride have also been used to provide short thickening times at the low temperatures involved. To obtain the required compressive strengths at the low temperatures, cement blends containing at least two different cements have heretofore been used. For example, U.S. Pat. No. 5,571,318 issued to Griffith et al. on Nov. 5, 1996 discloses cementing compositions for use in cold environments which are comprised of a relatively coarse particulate hydraulic cement mixed with an ultra fine particulate hydraulic cement. U.S. Pat. No. 5,806,594 issued to Stiles et al. on Sep. 15, 1998 discloses foamed cement compositions containing calcium sulfate cement and Portland cement.
API oil well cements are generally used in deep water offshore wells, and frequently, different classes of such cements must be used for cementing at different depths due to the temperature differences. The use of two or more different cements, for whatever purpose in drilling an offshore well, requires multiple cleanings of the bulk cement tanks located at the drilling rig platform as well as numerous trips to on-shore blending facilities for transporting the different cements to the platform. Thus, there is a need for a cement composition containing a single cement which can be used for cementing at various depths in an offshore well. The use of such a cement composition would be highly economical as well as environmentally preferable.
Another problem associated with offshore wells involves the use of dry additives such as set and compressive strength accelerating agents, set retarders, dispersing agents and the like in the cement compositions used. The use of dry additives requires the cement to be dry blended with the dry additives on-shore and the resulting blend to be transferred to the offshore platform where the dry blend is mixed with seawater. The blends can generally be used only for cementing at certain depths due to the differences in temperatures, fracture gradients and water flows. As a result, unused portions of specific cement blends have to be discarded and replaced with newly prepared blends. Again, this procedure wastes time and money in that it involves additional trips between the on-shore blending facility and the drilling platform. etc.
Thus, if only one basic cement is utilized and if the various additives are added in liquid form to the mix water or slurry on the offshore platform just prior to cementing, the bulk-cement inventory will be limited to one dry cement on the platform that can be used throughout the completion of the well, decisions on the precise nature of the cement composition to be used can be deferred until it is time to begin the cementing process and the operator will use only as much cement and liquid additives needed without generating costly waste and lost time.
Another problem which has often heretofore been encountered in deep water offshore well cementing is the inflow of formation fluids, e.g., water, gas and/or oil, into a cement composition which has been placed in the annulus between a casing string and the walls of a well bore, but which has not yet set. Such inflow of formation fluids is generally caused by the cement composition becoming partially self-supporting and the consequent loss of the ability of the cement column in the annulus to transmit hydrostatic pressure. When the pressure exerted by the cement composition falls below the pressure of formation fluids, the formation fluids enter the annulus and flow through the cement composition whereby flow channels are formed therein which remain after the cement composition is completely set.
The loss of the ability of the cement column to transmit hydrostatic pressure can result from the cement composition having too long a transition time, i.e., the time interval between when the cement composition begins to develop static gel strength and when the cement composition has sufficient gel strength to prevent the inflow of formation fluids. At the low temperatures encountered in offshore wells, it has heretofore been difficult if not impossible to shorten the transition times of cement compositions enough to eliminate formation fluid inflow and the problems associated therewith.
Thus, there are needs for improved methods and cement compositions for cementing casing strings in offshore well bores whereby formation fluid inflow is prevented.